The invention relates to a display screen, in particular a color display screen for a cathode ray tube, having a phosphor coating comprising a phosphor composition of a phosphor provided with a coating which comprises one or more oxygen compounds of the elements zinc, aluminium, magnesium, calcium and barium as well as colloidal SiO.sub.2 particles.
The invention also relates to a phosphor composition of a phosphor provided with a coating comprising one or more oxygen compounds of the elements zinc, aluminum, magnesium, calcium and barium as well as colloidal SiO.sub.2 particles.
The invention further relates to a method of manufacturing a phosphor composition from a phosphor provided with a coating comprising one or more oxygen compounds of the elements zinc, aluminum, magnesium, calcium and barium as well as colloidal SiO.sub.2 particles.
Display screens of cathode ray tubes are provided on the inner surface with structured phosphor layers which exhibit a dot pattern or a stripe pattern. In the case of color display tubes, each phosphor element of the phosphor layer is composed of a triplet of phosphor elements which each emit one of the three primary colors red, blue and green. This structured phosphor layer is customarily provided on the screen glass by means of a photochemical process, in which phosphor suspensions are used in the wet-chemical variant, and phosphor lacquers are used in the dry variant. In the wet-chemical process for coating screen glass with phosphors, aqueous phosphor suspensions are used which comprise, for example, polyvinyl alcohol (PVA) as the binder and ammonium dichromate (ADC) as the light-sensitive component.
Phosphor coatings which are applied to display screens by means of the "flow coat" process must meet the following requirements: a high packing density of the phosphor material in the phosphor coating in order to obtain an optimum screen brightness, a good homogeneity of the phosphor coating, i.e. preventing structures by means of the flow behavior of the suspension, or preventing "pin holes", a good homogeneity of the thickness of the phosphor coating, a high precision of the projection of the shadow mask structure on the structured phosphor coating, a high definition of the individual phosphor element, a good adhesion of the phosphor elements on the screen glass, a low degree of color impurity of the phosphor coating of an emission color as well as a low degree of haziness on unexposed screen glass.
Examples of criteria for a good stability and processability of phosphor suspensions in the display-screen process are a good stability of the particle-size distribution when an electrolyte solution is added and an ultrasonic treatment is carried out, a high filterability over a prolonged period of time when the pH-values are critical, and a medium-compact to compact phosphor sediment after sedimentation for 24 hours.
When the phosphor particles are processed in a suspension and when the phosphor coating is formed on the display-screen glass, in the wet-chemical display-screen process, numerous properties of said phosphor particles are influenced substantially by the surface properties of the phosphor powders from which the phosphor suspensions are prepared. To control the surface properties of the phosphor powders in view of the display-screen process, it proved to be advantageous to provide the inorganic phosphor powders, after their preparation, with chemically deviant coatings.
In U.S. Pat. No. 5,366,834, a method of manufacturing a display screen for an X-ray tube is disclosed, in which the inner surface of a glass face plate is coated with a phosphor suspension, said phosphor comprising a) phosphor particles, b) a first layer which is formed on the phosphor particles in a quantity of 0.1 to 5 wt. %, calculated with respect to the weight of the phosphor particles, said layer being composed of an essentially uniform silicon-dioxide film, and c) a second layer in a quantity of 0.008 to 1.5 wt. %, calculated with respect to the weight of the phosphor particles, said layer being formed on the first layer and being composed, at least, of a metal selected from the group formed by zinc and aluminium, and an alkaline earth metal and, at least, an element selected from the group formed by colloidal silicon oxide, aluminum-oxide sol and titanium-oxide-sol, each having a particle size of 50 nanometers or less.
Nowadays, the quality of display screens is to meet ever higher requirements in terms of brightness, color purity, absence of haziness and definition of the structured phosphor coating.